1. Field of the Invention
The present invention relates to an optical head device, and more particularly to an optical head which uses an optical separator and which is used in an optical recording/reproducing device for reproducing data on a recording medium, or for recording/reproducing or recording/reproducing/erasing data onto the recording medium.
2. Description of the Related Art
In the field of optical recording, device such as an optical disk in which a data in a co-axial circular or spiral track formed on the disk is optically reproduced is already in practical and commercial use.
In recent years, with a demand for increasing the memory capacity, there are increasing necessities for a higher recording density and for a reduction in a size of a spot formed on the recording medium. The size of a spot is determined by the wavelength of the laser light source and the numerical aperture (NA) in the objective lens, but the methods for attaining a shorter wavelength in a semiconductor laser and for increasing the numerical aperture NA have reached their limits. Thus, in recent years, research has been in progress on a super-resolution technique for attaining a spot diameter smaller than that of the diffraction limit determined by the wavelength of the laser light source and NA of the objective lens. Prior art examples in which such a super-resolution technique is applied have been disclosed, for example, in Japanese Patent Application Kokai Publication No. Hei 1-315040 and in Japanese Patent Application Kokai Publication No. Hei 1-315041.
In the above disclosed examples, because the super-resolution sidelobe incident on the recording medium causes deterioration of reproduction signals, it is proposed for the optical head device to employ a slit, a pinhole, etc. at the reimaging position in the detection optics for attaining a good reproduction signal (Japanese Patent Application Kokai Publication Hei 2-12623 and Japanese Patent Application Kokai Publication Hei 2-12624). Also, in order to attain a good reproduction signal, it is proposed that the optical head device detect a main beam component and a sidelobe component independently from each other and that the sidelobe component be electrically removed from the main beam component (Japanese Patent Application Kokai Publication Hei 2-206035 and Japanese Patent Application Kokai Publication Hei 2-206036).
The shape of the spot produced in an optical head device employing the super-resolution technique (hereinafter referred to as "super-resolution optical head device") is explained with reference to FIGS. 1A-1C.
For the above super-resolution optical head device, the basic optical system configurations of the first prior art example disclosed in Japanese Patent Application Kokai Publication No. Hei 2-12624 and the second prior art example disclosed in Japanese Patent Application Kokai Publication No. Hei 2-206036 are now explained with reference to FIGS. 2A and 2B, and FIGS. 3A and 3B. Each of these examples of the super-resolution optical head devices shown in FIGS. 2A & 2B and 3A & 3B is an example in which the super-resolution effect is attained only in one direction of the spot illuminated on the recording medium (hereinafter referred to as "one-dimensional super-resolution"), but this can be applied to an example in which the super resolution effect is attained in two directions of the converged spot (hereinafter referred to as "two-dimensional super-resolution").
FIGS. 1A and 1B show results obtained by simulation of the spot profile by the two-dimensional super-resolution. FIG. 1A shows a spot profile formed on the recording medium, and FIG. 1B shows a shape and a component distribution of the spot formed on a pinhole (reimaging position).
Where the wavelength of the laser light source is set to 680 nm, the radiation angles to 8.degree..times.21.degree., the focal length of the objective lens to 4.1 mm, the NA to 0.55, the focal length of the collimating lens to 25.0 mm and the radius of the two-dimensional light shielding region to 1.4 mm, a spot size of 0.89.times.0.81 .mu.m (1/e.sup.2) can be obtained on the recording medium, thereby yielding a spot diameter reducing effect of about 80% with respect to 1.10.times.1.01 .mu.m of the spot diameter (1/e.sup.2) on a diffraction limit.
The spot on the recording medium in the above state is, as shown in FIG. 1A, constituted by the main beam 37 and the sidelobe 38 so that, when the data on the recording medium is reproduced by the main beam 37, the sidelobe 38 unavoidably reproduces other data positioned in front and rear, and right and left of the data, with the sidelobe 38 resulting as noise in the main beam 37.
Also, the distribution of components when the spot is reimaged on the pinhole is shown in FIG. 1B, in which the total component 39 on the pinhole which is the sum of the main beam 37 and the sidelobe 38 on the recording medium resembles the spot profile on the recording medium. When the size of the pinhole aperture is set to be approximately the same as that of the reimaged spot, a shielded light component 40b in the sidelobe component 40 cannot be transmitted through the pinhole aperture and is removed optically from reproduction signals. When the sidelobe component 40 is considered with this being separated into a transmitted light component 40a which is transmitted through the pinhole aperture and a shielded light component 40b which is shielded thereby, it is appreciated that almost all of the sidelobe component 40 becomes the shielded light component 40b so that the reproduction signals are efficiently improved by the insertion of the pinhole.
FIG. 2A shows an optical system configuration of the super-resolution optical head device of the first prior art example explained above.
In FIG. 2A, laser beams emitted from a laser light source 1 are made into parallel light beams by a collimating lens 2, and are then transmitted through a super-resolution modulator 50, a polarizing beamsplitter 4 and a quarter-wave plate 5, after which they are converged by an objective lens 6 so that a micro spot is formed on a recording medium 7. Data signal light beams reflected from the recording medium 7 are transmitted through the objective lens 6 and the quarter-wave plate 5, which are then reflected totally by the polarizing beamsplitter 4, and then are incident on a beamsplitter 8.
The data signal light incident on the beamsplitter 8 is divided into two beams. One beam of light is transmitted through the beamsplitter 8 and, after being converged by a condenser lens (reconverging lens) 12, is transmitted through a slit 51 placed at a reimaging position and is received by a photodetector 14. The other beam of light is reflected by the beamsplitter 8, and is converged by a condenser lens (reconverging lens) 9, and after astigmatism is generated by a cylindrical lens 10, is received by a photodetector 11.
When a light shielding plate, for example, is used as the super-resolution modulator 50, a light shielding region 52 is formed on a center portion of light beams 53 as shown in a shape 50a of the light shielding plate so that a main beam whose spot diameter is smaller than that of a diffraction limit and a sidelobe are formed on the recording medium 7.
As a method to remove the sidelobe component from the data signal light, a slit 51 having at its central portion a light transmitting region 54 and at its peripheral portion a light shielding region 55 as shown in a slit shape 51a is used at the reimaging position so that the sidelobe formed at the reimaging position is removed optically.
A shape of a light receiving portion in the photodetector 14 is shown in FIG. 2B. One light receiving portion 58 exists in the shape 56 of a light receiving portion and receives only the main beam transmitted through the slit 51 as a main beam spot 57.
FIG. 3A shows an optical system configuration of the super-resolution optical head device of the second prior art example explained above.
In FIG. 3A an optical arrangement from a laser light source 1 to an objective lens 6 is the same as that in the first prior art example so that explanation on the arrangement will be omitted.
Data signal light beams reflected from a recording medium 7 are transmitted through an objective lens 6 and a quarter-wave plate 5, and are totally reflected by a polarizing beamsplitter 4, and then are incident on a beamsplitter 8. The data signal light beams that were incident on the beamsplitter 8 are divided into two beams. One beam of light is transmitted through the beamsplitter 8, and is converged by a condenser lens 12, and then is received by a photodetector 14 placed at a reimaging position. The other beam of light is reflected by the beamsplitter 8, and is converged by a condenser lens 9, and after astigmatism is generated by a cylindrical lens 10, is received by a photodetector 11.
A portion for receiving light of differing shapes in the photodetector 14 is shown in FIG. 3B. Three light receiving portions exist in the shape 59 of the light receiving point. A main beam light receiving portion 61a receives a main beam spot 60a and sidelobe light receiving portions 61b receive sidelobe beam spots 60b.
In order to remove the sidelobe component more precisely than in the first prior art example, a signal of the main beam light receiving portion 61a and a signal of the sidelobe light receiving portions 61b in the photodetector 14 placed at the reimaging position are transmitted through amplifiers whose gains are different from each other and are canceled out so that mixture of the sidelobe component 40 into the center portion as shown in FIG. 1B is removed electrically.
The super-resolution optical head device employs, as a focusing method of light beams converged on the recording medium 7 by the objective lens 6 in FIG. 2A and FIG. 3A, a conventional astigmatism method in which astigmatism is generated in the light beams reflected from the recording medium 7 by means of a cylindrical lens 10 and the change in the intensity of the light incident on the photodetector 11 is detected, thereby detecting a focusing error signal.
As a method to cause the converged beams on the recording medium 7 to follow a predetermined track, a conventional push-pull method is employed in which the change in the intensity of the light at a far field in the photodetector 11 receiving light beams reflected from the recording medium 7 is detected so that a tracking error signal is detected.
The first prior art example of the super-resolution optical head device has a shortcoming in that, as shown in FIGS. 1A-1C, the sidelobe 38 formed on the recording medium becomes a sidelobe component 40 at the reimaging position, and the shielded light component 40b in the sidelobe component 40 is removed by the pinhole or the slit having at its center portion a light transmitting region, but the transmitted light component 40a therein is transmitted therethrough and received by the photodetector so that reproduction signal deterioration caused by the transmitted light component 40a cannot be prevented.
In the second prior art example of the super-resolution optical head device, where the focal length of the objective lens is set to 4.1 mm and the focal length of the condenser lens (converging lens) to 30 mm, the magnification of the optical system becomes as large as 7.3, and where a spot diameter on the recording medium is set to 0.8 .mu.m and the distance to a sidelobe is set to 0.9 .mu.m, the size of the main beam light receiving position becomes about 6 .mu.m, and the distance between the center of the main beam light receiving portion and that of the sidelobe light receiving portion becomes about 7 .mu.m. With the current technology, it is impossible to get a photodetector having this size of the light receiving portion. In addition, there is a defect in that the main beam light receiving portion is as close as about 1-2 .mu.m to the sidelobe light receiving portion so that beams illuminating the light receiving portion and the peripheral portion thereof go around to the light receiving portion so that sufficient response of the light receiving portion cannot be attained.